Boko Haram extremists are actively planning to kidnap western foreign workers in north-east Nigeria, according to the US and UK governments.
The extremists are said to be targeting foreigners in the Bama area of Borno state, close to the Cameroon border.
The Nigeria-based Boko Haram has been pushed out of strongholds by military efforts but continues to control parts of the country's north-east.
This has challenged aid groups' efforts to address a hunger crisis that the United Nations says has left 4.7 million people in urgent need of food aid.
Nigeria is part of what the UN has called the largest humanitarian crisis since the world body was founded in 1945. The World Food Programme has warned of aid cuts if more help does not arrive.